Insoluble Anode of -Lead Dioxide Coated on Titanium for Electrosynthesis of Sodium Perchlorate N

JOURNAL OF APPLIED ELECTROCHEMISTRY 18 (1988) SHORT COMMUNICATION

SHORT COMMUNICATION Insoluble anode of -lead dioxide coated on titanium for electrosynthesis of sodium perchlorate N. MUNICHANDRAIAH, S. SATHYANARAYANA Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India Received 8 June 1987; revised 21 July 1987

1. Introduction production by a gas analysis technique were carried out. Lead dioxide is an important electrode material due to The analytical procedures, electrochemical cell, its stability under aggressive conditions (such as acidity electrical circuit and the procedure for recording of the electrolyte and high anode potentials), high steady-state galvanostatic anodic polarization curves electronic conductivity and low cost of material. Two and current efficiency measurements were as reported major areas of applications of lead dioxide are lead- previously [5]. acid batteries and electrosynthesis. Lead dioxide exhibits different morphological forms of which the 3. Results and discussion orthorhombic a-form and the tetragonal//-form have been extensively studied. Methods of preparation 3.1. Preparation of a-PbO 2 (Ti) electrode of these two forms of lead dioxide as well as their characteristics have been reviewed [1, 2]. The conditions for preparation of the a-PbO2(Ti) elec- Generally, the preferred form of this electrode trode (see Section 2, Experimental details) were arrived material in lead-acid batteries, as well as for electro- at as a result of deposition of a-PbO2 on platinized synthesis applications, is the fl-form. In spite of the titanium substrates from alkaline plumbite solutions fact that a-PbO2 is superior to fi-PbO2 both in elec- of different compositions with different current den- tronic conductivity [1] as well as electrochemical stab- sities and bath temperatures etc. The a-PbO2 (Ti) elec- ility [3], no study appears to have been made to eval- trode consisting of a 1.5mm thick PbO 2 layer uate the suitability of a-PbO2 as an electrode material. prepared under these favourable conditions was simi- In this paper we report our results on the preparation lar in appearance to the fl-PbO2(Ti) electrode and the of an a-PbO2 electrode coated onto a titanium sub- adherence to the substrate, as well as the coherence strate (a-PbO2 (Ti)) and its use as an insoluble anode within the coating, were good. The a-lead dioxide for the electro-oxidation of sodium chlorate to sodium layer did not peel off when the electrodes were used for perchlorate. anodic perchlorate synthesis, in contrast to reports in the literature [6]. A mild stirring of the alkaline plum- 2. Experimental details bite solution was found to be essential during the PbO2 coating process, since, in the absence of such a stirring Both a-PbO~ (Ti) electrodes and//-PbO 2(Ti) electrodes formation took place of a loosely bound reddish were prepared for comparative studies. The//-PbO2 brown oxide of lead which subsequently became was electrolytically deposited on a platinized titanium detached from the anode. As the solubility of lead surface as reported earlier [4] from an acidic lead acetate in potassium hydroxide (4 M) is only about nitrate and copper nitrate bath in the absence of any 40 g 1-1 a fairly large quantity of the electrolyte (one surface active agent. The a-PbOz(Ti) electrode was liter for 8 cm 2 of electrode) was required to deposit a prepared by anodic electrodeposition under the follow- 1.5 mm thick a-PbO2 coating to minimize the depletion ing conditions: (i) electrolyte, potassium hydroxide of plumbite ions during the coating operation. When solution (4 M) saturated with lead acetate and filtered; the electrolysis was prolonged to almost complete (ii) bath temperature, ambient (25 _+ 5~ (iii) depletion of plumbite ion in solution, the c~-PbO2 coat- cathode material, low-carbon steel (area about 20 ing, already present on the anode, was found to times larger than that of the anode); (iv) anode, plat- be partially converted to reddish brown oxide, render- inized titanium strip (area about 8 cm2); (v) stirring, ing the electrode unsuitable for electrosynthetic mild stirring using a magnetic stirrer; (vi) anode cur- applications. rent density, initially 50mAcm -2 for two minutes, The current efficiency for the deposition of the then decreased to 10 mA cm 2 for a few hours until the a-PbO2 is lower than that for the deposition of the required thickness was obtained. //-PbO2 (60% for the a-PbO2 and 95% for the fl-PbO2). In order to characterize and evaluate the perfor- As a result, it requires a longer time to deposit the mance of the a-PbO 2(Ti) electrode in comparison with a-PbO2 than the time required for depositing the the fi-PbO2(Ti) electrode, powder X-ray diffraction /%PbO2 of comparable thickness. Nervertheless, this is studies, steady-state galvanostatic anodic polarization not a major disadvantage since the electrode is curves in sodium chlorate solutions and instantaneous prepared only once for use as an insoluble anode, for current efficiency measurements for perchlorate a duration of several months. 314 0021-891X/88 $03.00 + .12 1988 Chapman and Hall Ltd. SHORT COMMUNICATIONS 3 ! 5

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I I I 2.4 2.6 2.8 2.4 2.6 2.8 2.4 2.5 2.8 3.0 {og Anode current density, mA cm-2 Fig. I. Instantaneous current efficiency for anodic chlorate oxidation on (1) c~-PbO2(Ti ) electrode; (2) fi-PbO2(Ti ) electrode as a function of anode current density in a solution of (a) 1 M sodium chlorate; (b) 3.2 M sodium chlorate; (c) 6.5 M sodium chlorate at 60 _+ 2 ~ C and pH6.5 _+ 0.2.

The use of low carbon steel as cathode material on e-PbOa(Ti) electrode than on the fi-PbO2(Ti) elec- which the overvoltage for the hydrogen evolution trode in all the three electrolyte concentrations reaction is low, is expected to reduce the losses of studied. The polarization curves (Fig. 2) are non- ptumbite ion due to cathodic reduction, in addition to linear due to the interference by the oxygen evolution a reduction in cell voltage during electrolysis. reaction and by ohmic drop in the electrolyte [5]. It may be seen from the polarization curves that the 3.2. Characterization of c~-Pb02 ( Ti) electrode anode potential is less at the c~-PbO2 (Ti) electrode than at the fl-PbO2(Ti) electrode, which is an additional X-ray diffractograms of powdered samples of e-PbO2 advantage with e-PbO2(Ti) insoluble anodes when (freshly prepared as well as from an electrode used as used in perchlorate cells. A study of the literature on an insoluble anode) revealed that the samples did not the subject reveals that the oxygen evolution reaction contain any form of PbO 2 other than the a-form. This takes place at lower anode potentials than the anodic is in contrast to fl-PbO2 prepared fi'om an acidic oxidation of chlorate ion to perchlorate ion [8] while nitrate bath which contains the a-form as a minor the overpotential for oxygen evolution is less on component [4, 7]. c~-PbO2 than on fl-PbO2 [3]. These facts explain the Current efficiency data obtained with an e-PbO2 (Ti) present results, viz. the increase in current efficiency anode and a fl-PbO2(Ti) anode in sodium chlorate and the decrease in anode potentials for anodic oxi- solutions of t M, 3.2M and 6.5 M concentrations are dation of chlorate ion to perchlorate ion on e-PbO~ (Ti) presented in Fig. 1. The corresponding steady-state electrodes as compared to fl-PbO2(Ti) electrodes, as galvanostatic anodic polarization curves are shown in due to either an electrocatalysis of the anodic oxi- Fig. 2. dation of chlorate and/or electro-retardation of anodic It may be seen from Fig. 1, that the current effic- oxygen evolution on e-PbO2(Ti) electrode. iency for perchlorate synthesis is higher on the Continuous electrolysis of sodium chlorate starting 3.0 LLI (b) (.3 bO

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1 I I I I ilJll I I I I Illl i 1.4 J---L ~ ~JIll] 1 l._l_.J_~j 10 100 10 100 10 100 1000 Anode current density ( mAcm"2} Fig. 2. Anodic galvanostatic steady-state polarization curves for (1) ~-PbO 2 (Ti) electrode and (2) fl-PbO2(Ti ) electrode in a solution of (a) 1 M sodium chlorate; (b) 3.2 M sodium chlorate; (c) 6.5 M sodium chlorate, at 60 + 2 ~ C and pH 6.5 + 0.2. "SSCE', Calomel electrode with saturated sodium chloride. - - 316 N. MUNICHANDRAIAH AND S. SATHYANARAYANA

t00 Fig. 3. Instantaneous current efficiency for anodic chlorate oxidation during continuous electrolysis of a solution of sodium chlorate (initial concentration 6.5 M; concentration at the end of electrolysis 1 M) at 60 + 2~ pH 6.5 _ 0.2 80 and anode current density 0.5Acm -2 on (1) c~-PbO2(Ti) electrode and (2) /3-PbOz (Ti) electrode. 0 A 0 ~ (3 60 B

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, I , I , I 0 20 40 60 80 Time of Electrolysis(h) with 6.5M solution and reaching about 1 M con- Additional advantages in using an e-PbO2(Ti) elec- centration at the end of the electrolysis (Fig. 3) trode are increased current efficiency and decreased revealed that the e-PbO2(Ti) electrode performed overpotential for anodic oxidation of chlorate ion to marginally better than the//-PbO2 (Ti) electrode with perchlorate ion. regard to cumulative current efficiency. Thus the aver- age cumulative current efficiency obtained by estimat- Acknowledgement ing the initial and final concentrations of chlorate ion for two electrolytic runs with each type of anode was Financial support for this work by the Indian Space 63.5% at the c~-PbO2(Ti) electrode and 58.8% at the Research Organization through a RESPOND project /~-PbO2(Ti) electrode. is gratefully acknowledged. The rate of anodic oxidation of chlorate ion to perchlorate ion increases with an increase in ionic References strength of the electrolyte at a constant concentration of chlorate ion [5]. Therefore, the current efficiency, [1] J.P. Cart and N. A. Hampson, Chem. Rev. 72 (1972) 679. [2] H. Bode, 'Lead-acid Batteries' (translated by R. J. Brodd when the chlorate concentration dropped to 1 M and K. V. Kordesch), John Wiley & Sons, New York during continuous electrolysis in the electrolyte of (1977). ionic strength of 6.5M, was higher (Fig. 3) com- [3] P. Riietschi, R. T. Angstadt and B. D. Cahan, J. Electro- chem. Soc. 106 (1959) 547. pared to the current efficiency in 1 M sodium chlorate [4] N. Munichandraiah and S. Sathyanarayana, J. Appl. (Fig. 1(a)). The difference in current efficiencies for the Electroehem. 17 (1987) 22. e-PbO2(Ti) electrode and the /?-PbO2(Ti) electrode [5] N. Munichandraiah and S. Sathyanarayana, ibid. 17 (1987) 33. was also not substantial in the electrolyte of high ionic [6] H. Hamsah, A.T. Kuhn and T.H. Randles, 'Electro- strength (Fig. 3). chemistry of Lead', edited by A. T. Kuhn, Academic Press, New York (1979) p. 296. In conclusion it may be stated that chemical, mech- [7] W. Mindt, J. Electrochem. Soc. 116 (1969) 1076. anical and electrochemical stability of an c~-PbO2 (Ti) [8] K. Sugino and S. Aoyagi, ibid. 103 (1956) 166. electrode is as good as that of a/~-PbO2(Ti) electrode.